The present invention relates to a method of firing a heat consuming device such as a boiler or furnace in which combustion within the heat consuming device is supported by oxygen separated from air by an oxygen transport membrane. More particularly, the present invention relates to such a firing method in which the separated oxygen also supports combustion to heat an incoming air stream to the oxygen transport membrane and flue gases from the heat consuming device are recirculated to dilute the oxygen being fed to the heat consuming device.
Carbon dioxide emissions arising from the combustion of fossil fuels have been identified as major contributors to the increase in the level of greenhouse gases in the earth""s atmosphere. This is especially true for the combustion of coal because of the greater carbon content of coal as compared with other types of fuels. Additionally, plants employing coal firing, for instance older electrical utilities, can operate at a lower thermal efficiency than plants fired by liquid fuels to thereby inherently generate more carbon dioxide emissions than liquid fired plants.
Separation and subsequent sequestration of carbon dioxide has been identified as one possible solution for reducing global warming. Sequestration after separation is achieved by compressing the gas to a high pressure and injecting it into deep formations in the ground or the oceans. Unfortunately, common means for removing carbon dioxide from flue gases such as amine scrubbing are expensive. Combustion that relies on oxygen, produced by cryogenic or pressure swing adsorption separation plants, reduces the cost of separating carbon dioxide from the flue gases since the primary combustion product is water which can easily be condensed. However, the costs involved in separating oxygen by cryogenic distillation or pressure swing adsorption makes such practice economically unattractive.
Although the prior art does not directly address the problem outlined above, like considerations have been dealt with in other fields. For instance, in U.S. Pat. No. 5,976,223, an oxygen transport membrane reactor is disclosed that employs ceramic materials to separate oxygen from oxygen-containing feeds. Such ceramic materials, generally perovskites, when heated and subjected to an oxygen partial pressure differential, can function to separate the oxygen from an oxygen-containing feed.
In a well known manner, oxygen is ionized at a cathode side of a membrane formed by a ceramic that can function to separate oxygen. The oxygen ions are transported through the membrane to an anode side thereof. At the anode side of the membrane, the oxygen ions recombine by losing the electrons gained upon ionization. The electrons are then used to ionize oxygen at the cathode side. In certain types of ceramics, known as mixed conductors, both oxygen ions and electrons are conducted. In ceramics known as ionic conductors, only the oxygen ions are conducted and thus, separate electrical pathways are provided for conducting the electrons.
In U.S. Pat. No. 5,976,233, permeated oxygen is combusted with a fuel at the permeate or anode side of the membrane. This combustion of the fuel reduces the oxygen partial pressure at the anode side of the membrane by consuming the permeated oxygen. Carbon dioxide can be recovered from the permeate effluent.
In U.S. Pat. No. 5,888,272, the permeate side of an oxygen transport membrane reactor is purged with combustion products from a downstream process into which fuel is injected. Combustion of the fuel consumes some of the oxygen produced to heat the membrane and to increase the driving force of oxygen through the membrane. The combustion effluent is then introduced into a downstream burner and used to support combustion within the burner and thereby produce the combustion effluent to be recirculated.
U.S. Pat. No. 6,149,714 discloses purging the permeate side of an oxygen transport membrane reactor with a purge gas stream having a low oxygen concentration. This produces an oxidant that is used to support combustion of the fuel and thereby create combustion products. Water can be condensed out of the combustion products and carbon dioxide can be recovered therefrom.
In all of the foregoing references, the incoming air stream must be heated. This heating consumes fuel and thereby produces carbon dioxide. As will be discussed, the present invention provides an integration involving the use of an oxygen transport membrane for oxy-fuel combustion in a heat consuming device in which the air containing the oxygen to be separated is also preheated with an oxy-fuel combustion.
The present invention relates to a method of firing a heat consuming device. It is to be noted, that the term xe2x80x9cheat consuming devicexe2x80x9d as used herein and in the claims means any device that consumes heat such as a boiler or a furnace.
In accordance with the present invention, air is compressed to form a compressed air stream. After compression, the compressed air stream is heated to form a heated compressed air stream. The compressed air stream is heated at least in part by a first oxy-fuel combustion. As used herein and in the claims, the term xe2x80x9coxy-fuel combustionxe2x80x9d indicates a combustion that is supported by oxygen contained within a gaseous mixture that does not contain molecular nitrogen such as in air. The oxygen is separated from the heated compressed air stream by an electrochemical separation process involving oxygen ion transport through a ceramic material to produce an oxygen permeate stream and a retentate stream. The heat consuming device is fired by a second oxy-fuel combustion producing a carbon dioxide-containing flue gas.
The first and second oxy-fuel combustion is supported with oxygen contained in the oxygen permeate. The oxygen is introduced into the second oxy-fuel combustion as a diluted oxygen stream formed by diluting the oxygen permeate with a diluent formed at least in part by recycling part of the carbon dioxide-containing flue gas. A product stream is extracted from the heat consuming device that is formed from a remaining part of the carbon dioxide-containing flue gas. This product stream can then be used in the downstream process or, alternatively, water and carbon dioxide can be separated from the stream for sequestration of the carbon dioxide.
Preferably the oxygen content of the diluted oxygen stream is between about 10 volume percent and about 40 volume percent. More preferably, the oxygen content of the diluted oxygen stream is between about 15 volume percent and about 25 volume percent. This is especially important when retrofitting a heat consuming device.
The oxygen can be separated from the heated compressed air stream within at least one oxygen transport membrane having a retentate side and a permeate side. At least part of the flue gas stream is formed from the part of the carbon dioxide-containing flue gas. The flue gas stream is introduced to the permeate side of the at least one oxygen transport membrane as a sweep gas stream, thereby to form an oxygen-containing sweep gas stream. The oxygen-containing sweep gas stream is introduced into a fired heater to support the first oxy-fuel combustion with a portion of the oxygen contained therein. This produces a combustion product stream. The diluted oxygen-containing stream is formed at least in part by the combustion product stream.
In another embodiment employing at least one oxygen transport membrane and a fired heater, a sweep gas stream is introduced to the permeate side of the at least one oxygen transport membrane to form an oxygen-containing sweep gas stream. Part of the oxygen-containing sweep gas stream and at least part of a flue gas stream, formed from the part of the carbon dioxide-containing flue gas, are introduced into the combustion chamber of the fired heater. This supports the first oxy-fuel combustion and forms the sweep gas stream. The diluted oxygen-containing stream is formed at least in part from a remaining part of the oxygen-containing sweep gas stream.
The preheating of the air can be carried in an oxygen transport membrane combustor-heater. This type of device is illustrated in U.S. Pat. No. 5,820,654. The oxygen is separated from the heated compressed air stream within first and second separations occurring in an oxygen transport membrane separator and an oxygen transport membrane combustor-heater, respectively. Each of the oxygen transport membrane and the oxygen transport membrane combustor-heater has opposed retentate and permeate sides. The compressed air stream is heated and the oxygen transport membrane combustor-heater in a heat exchanger located at the retentate side thereof. The first oxy-fuel combustion comprises a combustion of a fuel within the permeate side of the oxygen transport membrane combustor-heater.
In an embodiment of the present invention that employs an oxygen transport membrane combustor-heater, the fuel stream and at least part of a flue gas stream formed from the part of the carbon dioxide-containing flue gas, are introduced to the permeate side of the oxygen transport membrane combustor-heater as a reactive purge to react with a portion of the oxygen permeate, thereby to produce the first oxy-fuel combustion and a combustion product stream. The first separation produces an intermediate retentate stream that is in turn introduced into the retentate side of the oxygen transport membrane combustor-heater, thereby to affect the second separation and to form the retentate stream. The combustion product stream is introduced to the permeate side of the oxygen transport membrane separator, thereby to form an oxygen-containing combustion product stream. The diluted oxygen-containing stream is formed at least in part by the oxygen-containing combustion product stream.
In an alternative embodiment, the fuel stream, along with at least part of the flue gas stream, formed from at least a portion of the part of the carbon dioxide-containing flue gas, is introduced to the permeate side of the oxygen transport membrane combustor-heater to react with a portion of the oxygen permeate. This produces the first oxy-fuel combustion and a combustion product stream. The first separation produces an intermediate retentate stream. The intermediate retentate stream is expanded with the performance of work, thereby to produce a retentate exhaust stream. The retentate exhaust stream is introduced into the retentate side of the oxygen transport membrane combustor-heater thereby to affect the second separation and to form the retentate stream which can be exhausted from the system after recovery of contained heat or recovered as a nitrogen enriched product. The combustion product stream is introduced to the permeate side of the oxygen transport membrane separator, thereby to form an oxygen-containing combustion product stream. The diluted oxygen-containing stream is formed at least in part by the oxygen-containing combustion product stream.
In a further embodiment the permeate side of the combustor-heater is operated at an elevated pressure. In this regard, such embodiment as well as other embodiments employing a combustor-heater exploit the ability of an oxygen transport membrane to separate and transport oxygen from a lower total pressure to a higher total pressure when the partial oxygen pressure on the retentate side is greater than the oxygen partial pressure on the permeate side. The combustion product stream is withdrawn from the permeate side of the combustor-heater and expanded with the performance of work. Thereafter, the stream is introduced into a flue gas stream formed from at least a portion of the part of the carbon dioxide-containing flue gas. A fuel stream is compressed to form a compressed fuel stream that is introduced to the permeate side of the oxygen transport membrane combustor-heater. The work of expansion is applied at least to the compression of the fuel. Excess power can be used for export power generation. At least part of the flue gas stream, after introduction of the combustion product stream, is heated by the first oxy-fuel combustion and then introduced to the permeate side of the oxygen transport membrane separator, thereby to form an oxygen-containing combustion product stream. The first separation produces an intermediate retentate stream that is expanded with the performance of work to produce a retentate exhaust stream. The retentate exhaust stream is introduced to the retentate side of the oxygen transport membrane combustor-heater, thereby to affect a second separation to form the retentate stream. The diluted oxygen-containing stream is formed at least in part by the oxygen-containing combustion product stream. In such embodiment, the flue gas stream can be formed from the portion of the part of the carbon dioxide-containing flue gas. A further flue gas stream can also be formed from a remaining portion of the part of the carbon dioxide-containing flue gas and the further flue gas stream can be combined with the fuel gas stream prior to compression of the fuel gas stream.
In a yet further embodiment, a combustion product stream can be removed from the permeate side of the oxygen transport membrane combustor-heater and passed into indirect heat exchange with the fuel stream. The combustion product stream can be cooled and water can be separated thereby. Water can be separated from the product stream and after the water separation, the product stream is compressed to form a compressed product stream. The combustion product stream can be introduced into the compressed product stream. At least part of a flue gas stream formed from the part of the oxygen-containing flue gas, is heated by the first oxy-fuel combustion and introduced to the permeate side of the oxygen transport membrane separator as a sweep gas stream. This forms an oxygen-containing sweep gas stream. The first separation produces an intermediate retentate stream that is introduced to the retentate side of the oxygen transport membrane combustor-heater. This affects the second separation and produces the retentate stream. The diluted oxygen-containing stream is formed at least in part by the oxygen-containing sweep gas stream.
In all of the various embodiments of the present invention, heat and energy can be recaptured from the retentate stream and applied to partially heating the compressed air stream and to compress the incoming air stream. In embodiments of the present invention that utilize a fired heater and in the embodiment in which the intermediate retentate stream is introduced in the oxygen transport membrane combustor-heater, the retentate stream can be expanded with the performance of work. The work of expansion can be applied at least to the compression of the air stream. Excess power can be used to generate export power. An exhaust stream, composed of the retentate stream after expansion, can be passed in indirect heat exchange with the compressed air stream to partially heat the compressed air stream. In those embodiments of the present invention in which the intermediate retentate stream is expanded, the work of the expansion can be applied to the compression of the air stream or to generate export power. The retentate stream can be passed in indirect heat exchange with a compressed air stream to partially heat the compressed air stream.
In all embodiments of the present invention, the flue gas stream can be divided into first and second subsidiary flue gas streams. The at least part of the flue gas stream is the first subsidiary flue gas stream. The diluted oxygen-containing stream is also formed from the second subsidiary flue gas stream.
Additionally, the carbon dioxide-containing flue gas can be removed from a stack of the heat consuming device as a stream of the carbon dioxide-containing flue gas. The carbon dioxide-containing flue gas stream can be passed in indirect heat exchange with the compressed air stream and then divided into the flue gas stream and the product stream. The flue gas stream can be reheated in a recuperative heat exchanger located in the stack of the heat consuming device. In all cases, the second oxy-fuel combustion can be combustion of either coal or fuel oil. The heat consuming device can be a boiler.